This application is based upon and claims the benefit of priority from the prior Japanese Patent Applications No. 11-166673, filed Jun. 14, 1999; No. 11-238707, filed Aug. 25, 1999; No. 11-262216, filed Sep. 16, 1999; No. 11-291229, filed Oct. 13, 1999; and No. 11-312400, filed Nov. 2, 1999, the entire contents of which are incorporated herein by reference.
The present invention relates to a method of fabricating a thin-film photovoltaic module and, more particularly, to a method of fabricating a thin-film photovoltaic module by which a thin film is divided by laser scribing.
Generally, a thin-film photovoltaic module has a structure in which strip-like thin-film photovoltaic cells are formed on a substrate and connected in series. FIG. 1 is a sectional view schematically showing such a conventional module.
A module 1 shown in FIG. 1 is primarily composed of a glass substrate 2 and thin-film photovoltaic cells 10. Each thin-film photovoltaic cell 10 has a structure in which a transparent front electrode layer 3, a thin-film photovoltaic semiconductor layer 4, and a metal back electrode layer 5 are sequentially stacked on the glass substrate 2. The electrode layer 5 of each cell 10 is electrically connected to the electrode layer 3 of a cell 10 on its right side in FIG. 1. That is, these cells 10 are connected in series.
The electrode layer 3 electrically connected to the electrode layer 5 of the cell 10 on the right-hand end in FIG. 1 forms an output bus electrode area. The electrode layer 3 of the cell on the left-hand end in FIG. 1 extends outward, and this extending portion also forms an output bus electrode area. Electrode bus bars 12 such as copper foils are attached to these output bus electrode areas by ultrasonic solder 13 or the like.
This module 1 is usually fabricated as follows. First, a transparent front electrode layer 3 is formed as a thin continuous film on a glass substrate 2. Trenches 21 about, e.g., 50 xcexcm wide are formed by laser scribing to obtain a plurality of strip-like electrode layers 3. Next, a thin-film photovoltaic semiconductor layer 4 is formed as a thin continuous film on these electrode layers 3. In this layer 4 formed as a thin continuous film, trenches 22 for connecting the electrode layers 3 to electrode layers 5 are formed to have a width of, e.g., 100 xcexcm by laser scribing. Additionally, a metal back electrode layer 5 is formed as a thin continuous film on the layer 4. Trenches 23 about, e.g., 100 xcexcm thick are formed in this thin continuous film by laser scribing to obtain a plurality of strip-like electrode layers 5. After that, electrode bus bars 12 or the like are attached.
As described above, in the fabrication process of the module 1, thin-film division by laser scribing is performed whenever a thin-film is formed. In this laser scribing thin-film division, slight errors are inevitably produced in the positions of the trenches 21 to 23 because the large-area substrate 2 must be moved at high speed in a plane perpendicular to the optical axis of a laser beam. When such errors are produced, e.g., when errors are produced in the positions of the trenches 21 or 22, the electrode layers 3 and 5 may short in a single cell 10. When errors are produced in the positions of the trenches 22 or 23, the electrode layer 5 of one cell 10 may be incompletely connected to the electrode layer 3 of an adjacent cell 10. Hence, these trenches 21 to 23 are conventionally formed well apart from each other.
When the trenches 21 to 23 are thus formed sufficiently apart from each other, however, the width of each isolation region 15 for separating these thin-film photovoltaic cells 10 increases. Presently, the width of this isolation regions 15 is approximately 500 xcexcm, about twice as large 250 xcexcm, the sum of the widths of the trenches 21 to 23 described above. This isolation region 15 does not contribute to the generation of electric power, although it occupies part of the light-receiving surface of the module 1. Therefore, to obtain large output from the module 1 without sacrificing the fabrication yield, it is desired to reduce the width of the isolation region 15, i.e., to improve the accuracy of laser scribing.
Recently, the positional accuracy of light emission has greatly improved by checking the accuracy of a laser beam machine and precisely controlling its operation. Therefore, the width of an isolation region is expected to be largely reduced. In reality, however, even when it is attempted to reduce the width of an isolation region, the spacing between adjacent trenches in a single isolation region becomes much narrower than the designed value. In the worst case, these trenches overlap each other.
The present inventors investigated such defects and have found that the defects occur primarily in the vicinity of a position last irradiated with a laser beam in a thin-film photovoltaic semiconductor layer formed by CVD. From this tendency, the present inventors postulated that the defects are based not on the accuracy of a laser beam machine or the like, but on thermal expansion or thermal shrinkage of a glass substrate. As a result, the present inventors have found that the defects can indeed be prevented when the temperature of a glass substrate is controlled during laser scribing.
According to the present invention, there is provided a method of fabricating a thin-film photovoltaic module, including the following steps. A first electrode layer is formed on a substrate. The first electrode layer is divided by irradiating the first electrode layer with a laser beam in accordance with a first scanning pattern. The first scanning pattern is determined by taking into account a size of the substrate at a first target temperature. A thin-film photovoltaic semiconductor layer is formed on the divided first electrode layer. The thin-film photovoltaic semiconductor layer is divided by irradiating the thin-film photovoltaic semiconductor layer with a laser beam in accordance with a second scanning pattern. The second scanning pattern is determined by taking account of a size of the substrate at a second target temperature. A second electrode layer is formed on the divided thin-film photovoltaic semiconductor layer. The second electrode layer is divided by irradiating the second electrode layer with a laser beam in accordance with a third scanning pattern. The third scanning pattern is determined by taking account of a size of the substrate at a third target temperature. In each of the dividing steps, irradiation with the laser beam is performed under temperature-controlled conditions such that a difference between the target temperature and the temperature of the substrate is in a range from xe2x88x9210xc2x0 C. to +10xc2x0 C.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.